function y= trapezoidal(T,a,k)
acu = sol_acu(T,a);
t=0;
u=a;
temp1=a;
N = T/k;
for i=1:N
t = t + k;
flag = 0;
diff = 1000;
 while (flag<8)&&(abs(diff)>1e-8)
    temp0 = temp1;
    diff = (u + (k/2.0)*(fstif(temp0,t) + fstif(u,t-k))- temp0)/(1+(k*1e6/2.0));
    temp1 = temp0 + diff;
    flag = flag+1;
 end
 u = temp1;
end
y=abs(u-acu);
end